Any idea what this Canon lens cap is for?

A reader sent me this picture of what appears to be a smaller Canon lens cap. Does anyone have an idea what kind of Canon product uses this type? The size looks like 2/3 of a regular Canon lens cap. Could this be for Canon's mirrorless system?

Update: this is a rear cap from a Canon video lens. Sorry, false alarm.

Canon EF mount – 54mm, Canon XL mount – 42 mm… looks about right to me. What you got there is the back cap to a XL1 or XL2 lens just as folks have been saying. Its been at least 10 years since I’ve used one, but it sure looks familiar.